diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 493b97fde..7bec230ba 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -52,7 +52,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           # Should be the higest supported version, so we can use the newest tools
-          php-version: '8.3'
+          php-version: '8.4'
           tools: composer, composer-require-checker, composer-unused, phpcs
           extensions: ctype, date, dom, filter, hash, mbstring, openssl, pcre, soap, spl, xml
           coverage: none
@@ -145,7 +145,7 @@ jobs:
       fail-fast: false
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ['8.1', '8.2', '8.3']
+        php-versions: ['8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Setup PHP, with composer and extensions
@@ -185,15 +185,15 @@ jobs:
         run: composer install --no-progress --prefer-dist --optimize-autoloader
 
       - name: Run unit tests with coverage
-        if: ${{ matrix.php-versions == '8.3' }}
+        if: ${{ matrix.php-versions == '8.4' }}
         run: vendor/bin/phpunit
 
       - name: Run unit tests (no coverage)
-        if: ${{ matrix.php-versions != '8.3' }}
+        if: ${{ matrix.php-versions != '8.4' }}
         run: vendor/bin/phpunit --no-coverage
 
       - name: Save coverage data
-        if: ${{ matrix.php-versions == '8.3' }}
+        if: ${{ matrix.php-versions == '8.4' }}
         uses: actions/upload-artifact@v4
         with:
           name: coverage-data
@@ -207,7 +207,7 @@ jobs:
       fail-fast: true
       matrix:
         operating-system: [windows-latest]
-        php-versions: ['8.1', '8.2', '8.3']
+        php-versions: ['8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Setup PHP, with composer and extensions
diff --git a/src/Binding.php b/src/Binding.php
index 063b137aa..e4ca793d7 100644
--- a/src/Binding.php
+++ b/src/Binding.php
@@ -161,7 +161,7 @@ public function getDestination(): ?string
      *
      * @param string|null $relayState The RelayState.
      */
-    public function setRelayState(string $relayState = null): void
+    public function setRelayState(?string $relayState = null): void
     {
         $this->relayState = $relayState;
     }
@@ -185,7 +185,7 @@ public function getRelayState(): ?string
      *
      * @param string|null $destination The destination the message should be delivered to.
      */
-    public function setDestination(string $destination = null): void
+    public function setDestination(?string $destination = null): void
     {
         $this->destination = $destination;
     }
diff --git a/src/Certificate/KeyLoader.php b/src/Certificate/KeyLoader.php
index a2d14faa6..444e33b64 100644
--- a/src/Certificate/KeyLoader.php
+++ b/src/Certificate/KeyLoader.php
@@ -46,7 +46,7 @@ public function __construct()
      */
     public static function extractPublicKeys(
         CertificateProvider $config,
-        string $usage = null,
+        ?string $usage = null,
         bool $required = false,
     ): KeyCollection {
         $keyLoader = new self();
@@ -63,7 +63,7 @@ public static function extractPublicKeys(
      */
     public function loadKeysFromConfiguration(
         CertificateProvider $config,
-        string $usage = null,
+        ?string $usage = null,
         bool $required = false,
     ): KeyCollection {
         $keys = $config->getKeys();
@@ -96,7 +96,7 @@ public function loadKeysFromConfiguration(
      * @param array|\Traversable $configuredKeys
      * @param string|null $usage
      */
-    public function loadKeys($configuredKeys, string $usage = null): void
+    public function loadKeys($configuredKeys, ?string $usage = null): void
     {
         foreach ($configuredKeys as $keyData) {
             if (isset($keyData['X509Certificate'])) {
diff --git a/src/Certificate/PrivateKey.php b/src/Certificate/PrivateKey.php
index 2ced7f246..d775bf184 100644
--- a/src/Certificate/PrivateKey.php
+++ b/src/Certificate/PrivateKey.php
@@ -12,7 +12,7 @@ class PrivateKey extends Key
      * @throws \SimpleSAML\SAML2\Exception\InvalidArgumentException
      * @return \SimpleSAML\SAML2\Certificate\PrivateKey
      */
-    public static function create(string $keyContents, string $passphrase = null): PrivateKey
+    public static function create(string $keyContents, ?string $passphrase = null): PrivateKey
     {
         $keyData = ['PEM' => $keyContents, self::USAGE_ENCRYPTION => true];
         if ($passphrase) {
diff --git a/src/Compat/AbstractContainer.php b/src/Compat/AbstractContainer.php
index 1fe05de2b..af45888e1 100644
--- a/src/Compat/AbstractContainer.php
+++ b/src/Compat/AbstractContainer.php
@@ -177,7 +177,7 @@ abstract public function getTempDir(): string;
      * @param string $data The data we should write to the file.
      * @param int|null $mode The permissions to apply to the file. Defaults to 0600.
      */
-    abstract public function writeFile(string $filename, string $data, int $mode = null): void;
+    abstract public function writeFile(string $filename, string $data, ?int $mode = null): void;
 
 
     /**
diff --git a/src/Compat/MockContainer.php b/src/Compat/MockContainer.php
index b60018a54..4806f811d 100644
--- a/src/Compat/MockContainer.php
+++ b/src/Compat/MockContainer.php
@@ -61,7 +61,7 @@ public function debugMessage($message, string $type): void
      * @return string
      */
     public function getPOSTRedirectURL(
-        /** @scrutinizer ignore-unused */string $url = null,
+        /** @scrutinizer ignore-unused */?string $url = null,
         /** @scrutinizer ignore-unused */array $data = [],
     ): string {
         return strval($url);
@@ -82,7 +82,7 @@ public function getTempDir(): string
      * @param string $data
      * @param int|null $mode
      */
-    public function writeFile(string $filename, string $data, int $mode = null): void
+    public function writeFile(string $filename, string $data, ?int $mode = null): void
     {
         if ($mode === null) {
             $mode = 0600;
diff --git a/src/Configuration/DecryptionProvider.php b/src/Configuration/DecryptionProvider.php
index 5dd517dcb..8f580023c 100644
--- a/src/Configuration/DecryptionProvider.php
+++ b/src/Configuration/DecryptionProvider.php
@@ -24,7 +24,7 @@ public function getSharedKey(): ?string;
      *
      * @return mixed
      */
-    public function getPrivateKey(string $name, bool $required = null);
+    public function getPrivateKey(string $name, ?bool $required = null);
 
 
 
diff --git a/src/Configuration/IdentityProvider.php b/src/Configuration/IdentityProvider.php
index 03f9ac50e..646f9f23f 100644
--- a/src/Configuration/IdentityProvider.php
+++ b/src/Configuration/IdentityProvider.php
@@ -74,7 +74,7 @@ public function getSharedKey(): ?string
      * @param bool $required
      * @return mixed|null
      */
-    public function getPrivateKey(string $name, bool $required = null)
+    public function getPrivateKey(string $name, ?bool $required = null)
     {
         if ($required === null) {
             $required = false;
diff --git a/src/Configuration/ServiceProvider.php b/src/Configuration/ServiceProvider.php
index d2e92a2c6..352562e22 100644
--- a/src/Configuration/ServiceProvider.php
+++ b/src/Configuration/ServiceProvider.php
@@ -85,7 +85,7 @@ public function getSharedKey(): ?string
      * @param bool $required
      * @return mixed|null
      */
-    public function getPrivateKey(string $name, bool $required = null)
+    public function getPrivateKey(string $name, ?bool $required = null)
     {
         if ($required === null) {
             $required = false;
diff --git a/src/Exception/ProtocolViolationException.php b/src/Exception/ProtocolViolationException.php
index 99eb73d37..0b29fd4d2 100644
--- a/src/Exception/ProtocolViolationException.php
+++ b/src/Exception/ProtocolViolationException.php
@@ -12,9 +12,9 @@
 class ProtocolViolationException extends RuntimeException
 {
     /**
-     * @param string $message
+     * @param string|null $message
      */
-    public function __construct(string $message = null)
+    public function __construct(?string $message = null)
     {
         if ($message === null) {
             if (defined('static::DEFAULT_MESSAGE')) {
diff --git a/src/SOAPClient.php b/src/SOAPClient.php
index 6bea71169..96fc762c8 100644
--- a/src/SOAPClient.php
+++ b/src/SOAPClient.php
@@ -50,7 +50,7 @@ class SOAPClient
     public function send(
         AbstractMessage $msg,
         Configuration $srcMetadata,
-        Configuration $dstMetadata = null,
+        ?Configuration $dstMetadata = null,
     ): AbstractMessage {
         $issuer = $msg->getIssuer();
 
diff --git a/src/XML/EncryptedElementTrait.php b/src/XML/EncryptedElementTrait.php
index d773dd600..4b582b9d2 100644
--- a/src/XML/EncryptedElementTrait.php
+++ b/src/XML/EncryptedElementTrait.php
@@ -114,7 +114,7 @@ public static function fromXML(DOMElement $xml): static
     /**
      * @inheritDoc
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $this->encryptedData->toXML($e);
diff --git a/src/XML/ExtensionsTrait.php b/src/XML/ExtensionsTrait.php
index 6d3400c53..d4f5468c0 100644
--- a/src/XML/ExtensionsTrait.php
+++ b/src/XML/ExtensionsTrait.php
@@ -92,7 +92,7 @@ public function isEmptyElement(): bool
      * @param \DOMElement|null $parent The element we should add this Extensions element to.
      * @return \DOMElement The new md:Extensions XML element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/alg/DigestMethod.php b/src/XML/alg/DigestMethod.php
index b83519c29..f6d62ca75 100644
--- a/src/XML/alg/DigestMethod.php
+++ b/src/XML/alg/DigestMethod.php
@@ -80,7 +80,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Algorithm', $this->getAlgorithm());
diff --git a/src/XML/alg/SigningMethod.php b/src/XML/alg/SigningMethod.php
index 410fafd7e..3aa270ef0 100644
--- a/src/XML/alg/SigningMethod.php
+++ b/src/XML/alg/SigningMethod.php
@@ -113,7 +113,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/ecp/RelayState.php b/src/XML/ecp/RelayState.php
index e3c38a208..141a1c4b6 100644
--- a/src/XML/ecp/RelayState.php
+++ b/src/XML/ecp/RelayState.php
@@ -87,7 +87,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttributeNS(C::NS_SOAP_ENV_11, 'env:mustUnderstand', '1');
diff --git a/src/XML/ecp/Request.php b/src/XML/ecp/Request.php
index 1fe1270c5..a37b8abfb 100644
--- a/src/XML/ecp/Request.php
+++ b/src/XML/ecp/Request.php
@@ -152,7 +152,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttributeNS(C::NS_SOAP_ENV_11, 'env:mustUnderstand', '1');
diff --git a/src/XML/ecp/RequestAuthenticated.php b/src/XML/ecp/RequestAuthenticated.php
index 735dd2b3e..60b1d155b 100644
--- a/src/XML/ecp/RequestAuthenticated.php
+++ b/src/XML/ecp/RequestAuthenticated.php
@@ -98,7 +98,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/ecp/Response.php b/src/XML/ecp/Response.php
index 7c7a1f455..782c29e20 100644
--- a/src/XML/ecp/Response.php
+++ b/src/XML/ecp/Response.php
@@ -101,7 +101,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $response = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/ecp/SubjectConfirmation.php b/src/XML/ecp/SubjectConfirmation.php
index a83bf646c..b8578d843 100644
--- a/src/XML/ecp/SubjectConfirmation.php
+++ b/src/XML/ecp/SubjectConfirmation.php
@@ -122,7 +122,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this element to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttributeNS(C::NS_SOAP_ENV_11, 'env:mustUnderstand', '1');
diff --git a/src/XML/emd/RepublishRequest.php b/src/XML/emd/RepublishRequest.php
index 2d354550a..379b781b5 100644
--- a/src/XML/emd/RepublishRequest.php
+++ b/src/XML/emd/RepublishRequest.php
@@ -74,7 +74,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/md/AbstractEndpointType.php b/src/XML/md/AbstractEndpointType.php
index d02b9868a..0bb22e6d8 100644
--- a/src/XML/md/AbstractEndpointType.php
+++ b/src/XML/md/AbstractEndpointType.php
@@ -147,7 +147,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we should append this endpoint to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::instantiateParentElement($parent);
 
diff --git a/src/XML/md/AbstractIndexedEndpointType.php b/src/XML/md/AbstractIndexedEndpointType.php
index c3795048e..6a66a6ada 100644
--- a/src/XML/md/AbstractIndexedEndpointType.php
+++ b/src/XML/md/AbstractIndexedEndpointType.php
@@ -99,7 +99,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we should append this endpoint to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::instantiateParentElement($parent);
 
diff --git a/src/XML/md/AbstractLocalizedName.php b/src/XML/md/AbstractLocalizedName.php
index e208ac5c2..273e5051f 100644
--- a/src/XML/md/AbstractLocalizedName.php
+++ b/src/XML/md/AbstractLocalizedName.php
@@ -92,7 +92,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent
      * @return \DOMElement
      */
-    final public function toXML(DOMElement $parent = null): DOMElement
+    final public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttributeNS(C::NS_XML, 'xml:lang', $this->getLanguage());
diff --git a/src/XML/md/AbstractMetadataDocument.php b/src/XML/md/AbstractMetadataDocument.php
index 58d7cf262..29f8dfc5e 100644
--- a/src/XML/md/AbstractMetadataDocument.php
+++ b/src/XML/md/AbstractMetadataDocument.php
@@ -90,7 +90,7 @@ protected function getOriginalXML(): DOMElement
      *
      * @return \DOMElement
      */
-    public function toUnsignedXML(DOMElement $parent = null): DOMElement
+    public function toUnsignedXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/md/AbstractSSODescriptor.php b/src/XML/md/AbstractSSODescriptor.php
index 8b7382b66..19196fdfb 100644
--- a/src/XML/md/AbstractSSODescriptor.php
+++ b/src/XML/md/AbstractSSODescriptor.php
@@ -141,7 +141,7 @@ public function getNameIDFormat(): array
      * @param  \DOMElement|null $parent The EntityDescriptor we should append this SSODescriptorType to.
      * @return \DOMElement The generated SSODescriptor DOMElement.
      */
-    public function toUnsignedXML(DOMElement $parent = null): DOMElement
+    public function toUnsignedXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toUnsignedXML($parent);
 
diff --git a/src/XML/md/AbstractSignedMdElement.php b/src/XML/md/AbstractSignedMdElement.php
index af3f85ee7..378877d11 100644
--- a/src/XML/md/AbstractSignedMdElement.php
+++ b/src/XML/md/AbstractSignedMdElement.php
@@ -61,7 +61,7 @@ protected function setXML(DOMElement $xml): void
      * @return \DOMElement
      * @throws \Exception
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         if ($this->isSigned() === true && $this->signer === null) {
             // We already have a signed document and no signer was set to re-sign it
@@ -100,5 +100,5 @@ public function toXML(DOMElement $parent = null): DOMElement
      * @param  \DOMElement|null $parent
      * @return \DOMElement
      */
-    abstract public function toUnsignedXML(DOMElement $parent = null): DOMElement;
+    abstract public function toUnsignedXML(?DOMElement $parent = null): DOMElement;
 }
diff --git a/src/XML/md/AdditionalMetadataLocation.php b/src/XML/md/AdditionalMetadataLocation.php
index 968b7ed7a..af36afabd 100644
--- a/src/XML/md/AdditionalMetadataLocation.php
+++ b/src/XML/md/AdditionalMetadataLocation.php
@@ -90,7 +90,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement This AdditionalMetadataLocation-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getContent();
diff --git a/src/XML/md/AttributeConsumingService.php b/src/XML/md/AttributeConsumingService.php
index 2cf5ea8d4..a722f759a 100644
--- a/src/XML/md/AttributeConsumingService.php
+++ b/src/XML/md/AttributeConsumingService.php
@@ -151,7 +151,7 @@ public function getRequestedAttribute(): array
      * @param \DOMElement $parent The element we should append this AttributeConsumingService to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('index', strval($this->getIndex()));
diff --git a/src/XML/md/AuthnAuthorityDescriptor.php b/src/XML/md/AuthnAuthorityDescriptor.php
index fdc697599..8f7ee3513 100644
--- a/src/XML/md/AuthnAuthorityDescriptor.php
+++ b/src/XML/md/AuthnAuthorityDescriptor.php
@@ -44,7 +44,7 @@ public function __construct(
         array $protocolSupportEnumeration,
         protected array $assertionIDRequestService = [],
         protected array $nameIDFormat = [],
-        string $ID = null,
+        ?string $ID = null,
         ?DateTimeImmutable $validUntil = null,
         ?string $cacheDuration = null,
         ?Extensions $extensions = null,
diff --git a/src/XML/md/ContactPerson.php b/src/XML/md/ContactPerson.php
index 4e2d1197d..67e940ae3 100644
--- a/src/XML/md/ContactPerson.php
+++ b/src/XML/md/ContactPerson.php
@@ -203,7 +203,7 @@ public static function fromXML(DOMElement $xml): static
      *
      * @return \DOMElement The new ContactPerson-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/md/EntityDescriptor.php b/src/XML/md/EntityDescriptor.php
index 1c8a107f4..9224e4c3e 100644
--- a/src/XML/md/EntityDescriptor.php
+++ b/src/XML/md/EntityDescriptor.php
@@ -55,7 +55,7 @@ public function __construct(
         ?string $id = null,
         ?DateTimeImmutable $validUntil = null,
         ?string $cacheDuration = null,
-        Extensions $extensions = null,
+        ?Extensions $extensions = null,
         protected array $roleDescriptor = [],
         protected ?AffiliationDescriptor $affiliationDescriptor = null,
         protected ?Organization $organization = null,
diff --git a/src/XML/md/KeyDescriptor.php b/src/XML/md/KeyDescriptor.php
index e102c3df8..9e665c6bb 100644
--- a/src/XML/md/KeyDescriptor.php
+++ b/src/XML/md/KeyDescriptor.php
@@ -120,7 +120,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this KeyDescriptor to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/md/Organization.php b/src/XML/md/Organization.php
index 9cf41ab9c..6e3f96c4c 100644
--- a/src/XML/md/Organization.php
+++ b/src/XML/md/Organization.php
@@ -159,7 +159,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should add this organization to.
      * @return \DOMElement This Organization-element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/md/RequestedAttribute.php b/src/XML/md/RequestedAttribute.php
index 3b86eb7e2..38ffe3d6b 100644
--- a/src/XML/md/RequestedAttribute.php
+++ b/src/XML/md/RequestedAttribute.php
@@ -90,7 +90,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this RequestedAttribute to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toXML($parent);
 
diff --git a/src/XML/md/UnknownRoleDescriptor.php b/src/XML/md/UnknownRoleDescriptor.php
index 441def5a0..ce55c7b6e 100644
--- a/src/XML/md/UnknownRoleDescriptor.php
+++ b/src/XML/md/UnknownRoleDescriptor.php
@@ -81,7 +81,7 @@ public function getRawRoleDescriptor(): Chunk
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this unknown RoleDescriptor.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         return $this->getRawRoleDescriptor()->toXML($parent);
     }
diff --git a/src/XML/mdattr/EntityAttributes.php b/src/XML/mdattr/EntityAttributes.php
index d8993757d..66e3523a8 100644
--- a/src/XML/mdattr/EntityAttributes.php
+++ b/src/XML/mdattr/EntityAttributes.php
@@ -162,7 +162,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/mdrpi/Publication.php b/src/XML/mdrpi/Publication.php
index ef4d86460..daa7b8564 100644
--- a/src/XML/mdrpi/Publication.php
+++ b/src/XML/mdrpi/Publication.php
@@ -113,7 +113,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('publisher', $this->getPublisher());
diff --git a/src/XML/mdrpi/PublicationInfo.php b/src/XML/mdrpi/PublicationInfo.php
index a1b62b731..a33d06a1a 100644
--- a/src/XML/mdrpi/PublicationInfo.php
+++ b/src/XML/mdrpi/PublicationInfo.php
@@ -148,7 +148,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('publisher', $this->getPublisher());
diff --git a/src/XML/mdrpi/PublicationPath.php b/src/XML/mdrpi/PublicationPath.php
index b032c3c81..d959f88f6 100644
--- a/src/XML/mdrpi/PublicationPath.php
+++ b/src/XML/mdrpi/PublicationPath.php
@@ -82,7 +82,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/mdrpi/RegistrationInfo.php b/src/XML/mdrpi/RegistrationInfo.php
index 03b5089f3..54e3f7dd9 100644
--- a/src/XML/mdrpi/RegistrationInfo.php
+++ b/src/XML/mdrpi/RegistrationInfo.php
@@ -131,7 +131,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('registrationAuthority', $this->getRegistrationAuthority());
diff --git a/src/XML/mdui/DiscoHints.php b/src/XML/mdui/DiscoHints.php
index 3014f7a75..15bed14b8 100644
--- a/src/XML/mdui/DiscoHints.php
+++ b/src/XML/mdui/DiscoHints.php
@@ -152,7 +152,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/mdui/Keywords.php b/src/XML/mdui/Keywords.php
index a8cf1112f..070cea32a 100644
--- a/src/XML/mdui/Keywords.php
+++ b/src/XML/mdui/Keywords.php
@@ -106,7 +106,7 @@ public static function fromXML(DOMElement $xml): static
      * @throws \Exception
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         /** @psalm-var \DOMDocument $e->ownerDocument */
         $e = $this->instantiateParentElement($parent);
diff --git a/src/XML/mdui/Logo.php b/src/XML/mdui/Logo.php
index c82356487..0052584d5 100644
--- a/src/XML/mdui/Logo.php
+++ b/src/XML/mdui/Logo.php
@@ -129,7 +129,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Logo to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getContent();
diff --git a/src/XML/mdui/UIInfo.php b/src/XML/mdui/UIInfo.php
index e2b0a321a..bbb858e1f 100644
--- a/src/XML/mdui/UIInfo.php
+++ b/src/XML/mdui/UIInfo.php
@@ -302,7 +302,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AbstractBaseID.php b/src/XML/saml/AbstractBaseID.php
index 1a429bb51..4cb0d0fe7 100644
--- a/src/XML/saml/AbstractBaseID.php
+++ b/src/XML/saml/AbstractBaseID.php
@@ -122,7 +122,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this BaseID.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toXML($parent);
         $e->setAttributeNS(
diff --git a/src/XML/saml/AbstractBaseIDType.php b/src/XML/saml/AbstractBaseIDType.php
index 0484743c3..6e0536c68 100644
--- a/src/XML/saml/AbstractBaseIDType.php
+++ b/src/XML/saml/AbstractBaseIDType.php
@@ -42,7 +42,7 @@ protected function __construct(
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this BaseID.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AbstractCondition.php b/src/XML/saml/AbstractCondition.php
index 8a4af9c96..780ca6b2e 100644
--- a/src/XML/saml/AbstractCondition.php
+++ b/src/XML/saml/AbstractCondition.php
@@ -106,7 +106,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttributeNS(
diff --git a/src/XML/saml/AbstractStatement.php b/src/XML/saml/AbstractStatement.php
index 4139fe180..3a47cd395 100644
--- a/src/XML/saml/AbstractStatement.php
+++ b/src/XML/saml/AbstractStatement.php
@@ -106,7 +106,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Statement.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttributeNS(
diff --git a/src/XML/saml/Action.php b/src/XML/saml/Action.php
index 71b348dec..08f66e0d0 100644
--- a/src/XML/saml/Action.php
+++ b/src/XML/saml/Action.php
@@ -74,7 +74,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Action to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/Advice.php b/src/XML/saml/Advice.php
index 13afc08dd..b90c248c8 100644
--- a/src/XML/saml/Advice.php
+++ b/src/XML/saml/Advice.php
@@ -143,7 +143,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/Assertion.php b/src/XML/saml/Assertion.php
index f3230e26d..5e43e9713 100644
--- a/src/XML/saml/Assertion.php
+++ b/src/XML/saml/Assertion.php
@@ -341,7 +341,7 @@ protected function toUnsignedXML(?DOMElement $parent = null): DOMElement
      * @return \DOMElement This assertion.
      * @throws \Exception
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         if ($this->isSigned() === true && $this->signer === null) {
             // We already have a signed document and no signer was set to re-sign it
diff --git a/src/XML/saml/AssertionIDRef.php b/src/XML/saml/AssertionIDRef.php
index 1581e6fd9..5cd40e481 100644
--- a/src/XML/saml/AssertionIDRef.php
+++ b/src/XML/saml/AssertionIDRef.php
@@ -66,7 +66,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this AssertionIDRef.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $element = $this->instantiateParentElement($parent);
         $element->textContent = $this->getContent();
diff --git a/src/XML/saml/AssertionURIRef.php b/src/XML/saml/AssertionURIRef.php
index dca187c22..557595fd6 100644
--- a/src/XML/saml/AssertionURIRef.php
+++ b/src/XML/saml/AssertionURIRef.php
@@ -66,7 +66,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $element = $this->instantiateParentElement($parent);
         $element->textContent = $this->getContent();
diff --git a/src/XML/saml/Attribute.php b/src/XML/saml/Attribute.php
index 697e72717..52dc79ab1 100644
--- a/src/XML/saml/Attribute.php
+++ b/src/XML/saml/Attribute.php
@@ -151,7 +151,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Attribute to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Name', $this->getName());
diff --git a/src/XML/saml/AttributeStatement.php b/src/XML/saml/AttributeStatement.php
index 05553bb5a..74b26fa7a 100644
--- a/src/XML/saml/AttributeStatement.php
+++ b/src/XML/saml/AttributeStatement.php
@@ -86,7 +86,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Attribute to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AttributeValue.php b/src/XML/saml/AttributeValue.php
index 0b136f7d7..069bc7f01 100644
--- a/src/XML/saml/AttributeValue.php
+++ b/src/XML/saml/AttributeValue.php
@@ -156,7 +156,7 @@ public static function fromXML(DOMElement $xml): static
      *
      * @return \DOMElement The generated AttributeValue element.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::instantiateParentElement($parent);
 
diff --git a/src/XML/saml/Audience.php b/src/XML/saml/Audience.php
index 14aaf20b6..cf04ac29c 100644
--- a/src/XML/saml/Audience.php
+++ b/src/XML/saml/Audience.php
@@ -66,7 +66,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Audience.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $element = $this->instantiateParentElement($parent);
         $element->textContent = $this->getContent();
diff --git a/src/XML/saml/AudienceRestriction.php b/src/XML/saml/AudienceRestriction.php
index 0fbb4c1a3..7043c9ed6 100644
--- a/src/XML/saml/AudienceRestriction.php
+++ b/src/XML/saml/AudienceRestriction.php
@@ -68,7 +68,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this AudienceRestriction.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AuthnContext.php b/src/XML/saml/AuthnContext.php
index 9f63c6d90..5c1b4a04a 100644
--- a/src/XML/saml/AuthnContext.php
+++ b/src/XML/saml/AuthnContext.php
@@ -154,7 +154,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this AuthnContextDeclRef to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AuthnContextDecl.php b/src/XML/saml/AuthnContextDecl.php
index 79beda286..2630511c9 100644
--- a/src/XML/saml/AuthnContextDecl.php
+++ b/src/XML/saml/AuthnContextDecl.php
@@ -81,7 +81,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this AuthnContextDecl to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AuthnStatement.php b/src/XML/saml/AuthnStatement.php
index 8a275c123..17a76b240 100644
--- a/src/XML/saml/AuthnStatement.php
+++ b/src/XML/saml/AuthnStatement.php
@@ -166,7 +166,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this AuthnStatement to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/AuthzDecisionStatement.php b/src/XML/saml/AuthzDecisionStatement.php
index 9488a9305..04d56894e 100644
--- a/src/XML/saml/AuthzDecisionStatement.php
+++ b/src/XML/saml/AuthzDecisionStatement.php
@@ -145,7 +145,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this AuthzDecisionStatement to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/Conditions.php b/src/XML/saml/Conditions.php
index b873c2b43..506635c31 100644
--- a/src/XML/saml/Conditions.php
+++ b/src/XML/saml/Conditions.php
@@ -195,7 +195,7 @@ public static function fromXML(DOMElement $xml): static
      * @param  \DOMElement|null $parent The parent element we should append this element to.
      * @return \DOMElement This element, as XML.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/Evidence.php b/src/XML/saml/Evidence.php
index 8f189c6e4..01bded9e5 100644
--- a/src/XML/saml/Evidence.php
+++ b/src/XML/saml/Evidence.php
@@ -129,7 +129,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/NameIDType.php b/src/XML/saml/NameIDType.php
index 926fe3b9e..46f224970 100644
--- a/src/XML/saml/NameIDType.php
+++ b/src/XML/saml/NameIDType.php
@@ -111,7 +111,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this NameIDType.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/OneTimeUse.php b/src/XML/saml/OneTimeUse.php
index a9b357562..d22d0d750 100644
--- a/src/XML/saml/OneTimeUse.php
+++ b/src/XML/saml/OneTimeUse.php
@@ -39,7 +39,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this OneTimeUse.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         return $this->instantiateParentElement($parent);
     }
diff --git a/src/XML/saml/ProxyRestriction.php b/src/XML/saml/ProxyRestriction.php
index 7301d0ccd..7b8759ef5 100644
--- a/src/XML/saml/ProxyRestriction.php
+++ b/src/XML/saml/ProxyRestriction.php
@@ -79,7 +79,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/Subject.php b/src/XML/saml/Subject.php
index 175845b33..bb7c4ea1c 100644
--- a/src/XML/saml/Subject.php
+++ b/src/XML/saml/Subject.php
@@ -94,7 +94,7 @@ public static function fromXML(DOMElement $xml): static
      * @param  \DOMElement|null $parent The parent element we should append this element to.
      * @return \DOMElement This element, as XML.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/SubjectConfirmation.php b/src/XML/saml/SubjectConfirmation.php
index 790792d3f..7be8d92fd 100644
--- a/src/XML/saml/SubjectConfirmation.php
+++ b/src/XML/saml/SubjectConfirmation.php
@@ -105,7 +105,7 @@ public static function fromXML(DOMElement $xml): static
      * @param  \DOMElement|null $parent The parent element we should append this element to.
      * @return \DOMElement This element, as XML.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Method', $this->getMethod());
diff --git a/src/XML/saml/SubjectConfirmationData.php b/src/XML/saml/SubjectConfirmationData.php
index 88480ce11..a3a2b7bc2 100644
--- a/src/XML/saml/SubjectConfirmationData.php
+++ b/src/XML/saml/SubjectConfirmationData.php
@@ -202,7 +202,7 @@ public static function fromXML(DOMElement $xml): static
      * @param  \DOMElement|null $parent The parent element we should append this element to.
      * @return \DOMElement This element, as XML.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/SubjectLocality.php b/src/XML/saml/SubjectLocality.php
index b16194199..292bfbd81 100644
--- a/src/XML/saml/SubjectLocality.php
+++ b/src/XML/saml/SubjectLocality.php
@@ -91,7 +91,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this SubjectLocality to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/saml/UnknownCondition.php b/src/XML/saml/UnknownCondition.php
index 957a340c8..e2999da94 100644
--- a/src/XML/saml/UnknownCondition.php
+++ b/src/XML/saml/UnknownCondition.php
@@ -43,7 +43,7 @@ public function getRawCondition(): Chunk
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this unknown condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         return $this->getRawCondition()->toXML($parent);
     }
diff --git a/src/XML/saml/UnknownID.php b/src/XML/saml/UnknownID.php
index 876036b05..122d84c25 100644
--- a/src/XML/saml/UnknownID.php
+++ b/src/XML/saml/UnknownID.php
@@ -47,7 +47,7 @@ public function getRawIdentifier(): Chunk
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this unknown ID.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         return $this->getRawIdentifier()->toXML($parent);
     }
diff --git a/src/XML/saml/UnknownStatement.php b/src/XML/saml/UnknownStatement.php
index 1f352c4b4..0008fa1bc 100644
--- a/src/XML/saml/UnknownStatement.php
+++ b/src/XML/saml/UnknownStatement.php
@@ -43,7 +43,7 @@ public function getRawStatement(): Chunk
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this unknown statement.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         return $this->getRawStatement()->toXML($parent);
     }
diff --git a/src/XML/samlp/AssertionIDRequest.php b/src/XML/samlp/AssertionIDRequest.php
index fbe65d172..a6ad17789 100644
--- a/src/XML/samlp/AssertionIDRequest.php
+++ b/src/XML/samlp/AssertionIDRequest.php
@@ -158,7 +158,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this AssertionIDRequest element to.
      * @return \DOMElement
      */
-    public function toUnsignedXML(DOMElement $parent = null): DOMElement
+    public function toUnsignedXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toUnsignedXML($parent);
 
diff --git a/src/XML/samlp/IDPEntry.php b/src/XML/samlp/IDPEntry.php
index c69885dc2..9d6bf8bb3 100644
--- a/src/XML/samlp/IDPEntry.php
+++ b/src/XML/samlp/IDPEntry.php
@@ -97,7 +97,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this IDPEntry to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('ProviderID', $this->getProviderId());
diff --git a/src/XML/samlp/IDPList.php b/src/XML/samlp/IDPList.php
index 97831eed0..54d6d9f35 100644
--- a/src/XML/samlp/IDPList.php
+++ b/src/XML/samlp/IDPList.php
@@ -106,7 +106,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this IDPList to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/samlp/NameIDPolicy.php b/src/XML/samlp/NameIDPolicy.php
index 3b7a40f8e..cdc275caa 100644
--- a/src/XML/samlp/NameIDPolicy.php
+++ b/src/XML/samlp/NameIDPolicy.php
@@ -113,7 +113,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this NameIDPolicy to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/samlp/RequestedAuthnContext.php b/src/XML/samlp/RequestedAuthnContext.php
index 02c9900fa..af3ffc3d3 100644
--- a/src/XML/samlp/RequestedAuthnContext.php
+++ b/src/XML/samlp/RequestedAuthnContext.php
@@ -112,7 +112,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this RequestedAuthnContext to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/samlp/Scoping.php b/src/XML/samlp/Scoping.php
index fb3a0937e..7d63dff46 100644
--- a/src/XML/samlp/Scoping.php
+++ b/src/XML/samlp/Scoping.php
@@ -108,7 +108,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Scoping to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/samlp/Status.php b/src/XML/samlp/Status.php
index ce3d9e75b..e43c9df72 100644
--- a/src/XML/samlp/Status.php
+++ b/src/XML/samlp/Status.php
@@ -123,7 +123,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Status.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/samlp/StatusCode.php b/src/XML/samlp/StatusCode.php
index 5a2ed97c2..9b6919636 100644
--- a/src/XML/samlp/StatusCode.php
+++ b/src/XML/samlp/StatusCode.php
@@ -87,7 +87,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this StatusCode to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->setAttribute('Value', $this->getValue());
diff --git a/src/XML/samlp/StatusDetail.php b/src/XML/samlp/StatusDetail.php
index b1ffda576..24e058226 100644
--- a/src/XML/samlp/StatusDetail.php
+++ b/src/XML/samlp/StatusDetail.php
@@ -71,7 +71,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this StatusDetail.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/shibmd/KeyAuthority.php b/src/XML/shibmd/KeyAuthority.php
index 2e3cc424d..9e4c2bab6 100644
--- a/src/XML/shibmd/KeyAuthority.php
+++ b/src/XML/shibmd/KeyAuthority.php
@@ -99,7 +99,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Scope to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
 
diff --git a/src/XML/shibmd/Scope.php b/src/XML/shibmd/Scope.php
index 9fcdd71bb..e5b759d93 100644
--- a/src/XML/shibmd/Scope.php
+++ b/src/XML/shibmd/Scope.php
@@ -85,7 +85,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement|null $parent The element we should append this Scope to.
      * @return \DOMElement
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = $this->instantiateParentElement($parent);
         $e->textContent = $this->getContent();
diff --git a/tests/SAML2/CustomBaseID.php b/tests/SAML2/CustomBaseID.php
index e54cd43fe..ed64faba9 100644
--- a/tests/SAML2/CustomBaseID.php
+++ b/tests/SAML2/CustomBaseID.php
@@ -38,8 +38,8 @@ final class CustomBaseID extends AbstractBaseID
      */
     public function __construct(
         protected array $audience,
-        string $NameQualifier = null,
-        string $SPNameQualifier = null,
+        ?string $NameQualifier = null,
+        ?string $SPNameQualifier = null,
     ) {
         Assert::allIsInstanceOf($audience, Audience::class);
 
@@ -88,7 +88,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this BaseID.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toXML($parent);
 
diff --git a/tests/SAML2/CustomCondition.php b/tests/SAML2/CustomCondition.php
index a0889690b..6b88b7709 100644
--- a/tests/SAML2/CustomCondition.php
+++ b/tests/SAML2/CustomCondition.php
@@ -88,7 +88,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Condition.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toXML($parent);
 
diff --git a/tests/SAML2/CustomRoleDescriptor.php b/tests/SAML2/CustomRoleDescriptor.php
index 067089233..fba5d7403 100644
--- a/tests/SAML2/CustomRoleDescriptor.php
+++ b/tests/SAML2/CustomRoleDescriptor.php
@@ -174,7 +174,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this RoleDescriptor.
      */
-    public function toUnsignedXML(DOMElement $parent = null): DOMElement
+    public function toUnsignedXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toUnsignedXML($parent);
 
diff --git a/tests/SAML2/CustomStatement.php b/tests/SAML2/CustomStatement.php
index 019920964..edcba56f1 100644
--- a/tests/SAML2/CustomStatement.php
+++ b/tests/SAML2/CustomStatement.php
@@ -88,7 +88,7 @@ public static function fromXML(DOMElement $xml): static
      * @param \DOMElement $parent The element we are converting to XML.
      * @return \DOMElement The XML element after adding the data corresponding to this Statement.
      */
-    public function toXML(DOMElement $parent = null): DOMElement
+    public function toXML(?DOMElement $parent = null): DOMElement
     {
         $e = parent::toXML($parent);